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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

FATTY SUBSTANCES   (GENERAL METHODS)            385

for about 10 minutes with continual shaking, the lead soap becoming attached
to the sides and bottom of the flask, while the liquid becomes clear. The
whole of the liquid is then decanted off and the soap washed with three
successive quantities of 200 c.c. of hot water (70-80). The water is drained
off, the beaker cooled, the last drops of water adhering to the soap removed
by means of filter-paper, and 220 c.c. of ether added. The flask is well
shaken and then fitted with a reflux apparatus and the liquid gently boiled
on a water-bath for 20 minutes with occasional shaking and subsequently
immersed in cold water (4-5) for two hours. The clear ether is filtered
into a separating funnel,1 care being taken to let as little as possible of the
undissolved soap fall into the filter. The residue in the flask is then heated
for 20 minutes with a fresh quantity of 100 c.c. of ether under a reflux con-
denser and the flask afterwards stoppered and placed on one side immersed
in cold water.

Meanwhile the filter is washed with a little very cold ether which is
caught in the separating funnel containing the other filtrate. The well-
covered filter is placed on one side, while the ethereal liquid in the separating
funnel is vigorously shaken with 150 c.c. of 20% hydrochloric acid to decom-
pose the lead soap of the liquid acids and then left to stand until the ether
has collected at the surface in a clear layer. The lower aqueous layer,
together with the precipitated lead chloride, is run off, the treatment repeated
with 100 c.c. of hydrochloric acid, and the ethereal solution then washed
three times with distilled water (150 c.c. each time). Finally, the bulk of
the ether is distilled off and the last traces driven off on a water-bath while
a current of carbon dioxide is passed.

The liquid acids thus obtained are weighed and calculated to 100 parts
of the substance. These liquid acids, provided they are kept out of contact
with the air, may be further utilised for the detection of cottonseed oil
and sesame oil (q.v.} and for the determination of the absolute iodine number
(see 13, above).

The lead soap of the solid fatty acids, left undissolved in the flask im-
mersed in cold water, is collected on the filter placed on one side, as men-
tioned above, and washed well with very cold ether and then introduced
into a separating funnel where it is shaken with ether and hydrochloric acid
as in the case of the liquid acids.

The ethereal solution of the solid fatty acids is distilled and the residue
dried in an oven at 100 and weighed. For greater accuracy, the weight
found may be increased by the quantity of solid acids (stearic and palmitic)
corresponding with their lead soap remaining dissolved in the ether, knowing
that 50 c.c. of anhydrous ether at the ordinary temperature dissolve 0-0074
gram of lead stearate and 0-0092 gram of lead palmitate; these amounts
correspond with 0-0054 gram of stearic and 0-0065 gram of palmitic acid.
The solid acids thus separated may then be utilised for other investigations,
e.g., for that of the arachidic and lignoceric acids (see Arachis Oil).

Other gravimetric methods for the quantitative separation of the solid
from the liquid fatty acids are (i) those of David2 and Fakiola* based on

1  For very exact determinations the funnel should be cooled externally with  ice.

2  Ann. de chim. analyt., 1911, p. 8.      3 Gazz. cJiim. ital, 1910, II, pp. 217 and 425.

A.c.                                                                                             25	3-66